Funds are requested to upgrade existing X-ray diffraction instrumentation in the Research Triangle area in order to improve the quality and productivity rate of a number of on-going projects in structural macromolecular biology. Projects fall into two principal categories: - Studies of components and intermediate molecular complexes important to genetic recombination and translation, and to repair of DNA damage. These include: a. Analysis of oriented fiber samples of the recombination enzyme RecA and several RecA protein-DNA complexes prepared in the presence of different nucleotide substrates and substrate analogs, and with both single and double-stranded DNA; and similar studies using the single-stranded DNA binding protein SSB. b. Single crystal structure determinations of Tryptophanyl tRNA synthetase complexed to a substrate (tryptophan) and an acyl transfer product (tryptophanyl-3'[2']ATP), to elucidate the mechanism of its normal function and a possible abnormal function in carcinogen activation; and single crystal studies of proteins involved in repair of damaged DNA (UVR excinuclease, photolyase). - Studies in membrane biology, including complexes reconstituted from components of nerve myelin, rod outer segments from frog retina, X-ray analysis of morphological changes accompanying freeze-fracture sample preparation, and the membrane junction between lens fiber cells. These projects benefit from thorough biochemical characterization and preliminary structural studies. In projects from the former category, different complexes which appear to fit into a series of structural stages for the respective genetic processes have been prepared. Large strides toward elucidating the underlying physical mechanisms by comparing and ordering the complexes can be expected.